Wafer tray and ceramic blade for semiconductor processing apparatus

ABSTRACT

A semiconductor wafer processing system for processing wafers from a wafer storage cassette includes a wafer transfer chamber; a wafer storage elevator within the transfer chamber; one or more wafer processing chambers; and a wafer transfer apparatus for transferring a wafer between a standard storage cassette adjacent and outside the transfer chamber and the elevator, and between the elevator and the processing chamber. The storage chamber pressure varies between atmospheric when accepting wafers from outside, and a subatmospheric pressure when transferring wafers to or from a processing chamber. The transfer apparatus includes a robot arm; a thin flat wafer carrying blade at the leading end of the robot arm configured for engaging a wafer from the storage cassette or the elevator; and a wafer support tray configured for removable engagement with the blade and for engaging and positively positioning a wafer from the elevator, or a support pedestal within a processing chamber. When the transfer apparatus moves a wafer between the elevator and a processing chamber in an evacuated environment, the tray is engaged with the blade and helps retain the wafer during transit. When wafers are transferred between the cassette and the elevator at atmospheric pressure the tray is disengaged from the blade and placed in a rest position on the elevator, and the wafer transfer is performed by means of the blade alone with a vacuum pick integral to the blade. The blade includes upper and lower halves together defining vacuum channels and capacitive position sensors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 08/205,711, filedMar. 3, 1994 as a continuation-in-part application of application Ser.No. 08/093,236, filed on Jul. 15, 1993 and now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to semiconductor processing apparatus and moreparticularly to improved apparatus capable of transferring wafers withinan evacuated environment, or atmospheric environment, or between bothenvironments.

2. Brief Description of the Prior Art

Semiconductor wafers are typically processed in vacuum processingsystems. These systems include one or more chambers, each performingwafer processing operations such as etching, chemical vapor depositionor physical vapor deposition, which often require heating or cooling ofthe wafer, and a plasma to assist the process. Typically the environmentwithin such processing chambers is maintained at a low subatmosphericpressure. Each chamber includes inlets and outlets for evacuationapparatus and the admission of processing gases, as well as an aperturecontrolled by a slit valve to admit wafers. Such processing chambers mayin turn communicate with a wafer transfer chamber, and in turn thetransfer chamber will have a valve-controlled aperture by which waferscan be admitted from outside the system.

The transfer of a wafer to and from a chamber and to and from theoutside of the system is generally done mechanically by means of a robotarm at the end of which is a wafer retaining means. There are two typesof wafer retaining means used in the art. The first type is a flat bladethrough which a vacuum channel is formed, terminating in an outlet. Thisis so that the blade can pick up a wafer by touching the surfacecontaining the outlet, typically the upper surface of the blade, to thebottom surface of the wafer and applying a vacuum, so as to cause thewafer to stick to the blade. The advantage of the flat vacuum bladepickup is that the blade, being flat and thin, can be relatively easilymaneuvered between the tight spaces of a wafer storage cassette to pickup a wafer.

There are two prominent disadvantages to the vacuum blade. First, sincethe blade pickup function relies on vacuum suction to hold a wafer inplace, the blade pickup is completely ineffective for holding a wafer inan evacuated environment. Second, the construction of the vacuum bladeis expensive, and the blade can break down under high temperature orcorrosive conditions. The construction of the blade is typically anexpensive multilayer laminate of metal and plastic bonded together withsilicone rubber. These layers tend to separate or warp when contactedwith a hot wafer, and the blade can corrode in the presence of corrosivewafer processing gases.

To overcome the disadvantages of the flat vacuum blade, a shoe attachedto the robot arm has been used. This second prior art wafer retainingsurface comprises a shoe, or tray-like extension at the leading end ofthe arm, having a bevelled contour shaped to accommodate a wafer. Theshoe helps to engage the wafer and retain the wafer in place upon thearm while the robot arm swings around to deliver the wafer to anotherlocation.

Although the shoe is fairly effective for moving wafers in a vacuumenvironment, it is less effective and efficient in the ambientatmospheric environment, particularly where wafers must be transferredto and from a standard wafer cassette. One drawback is that a vacuumpick on the shoe is typically still necessary to transfer wafers from awafer cassette. In designs in which the arm to which the shoe isattached extends to the bottom center of the blade, only the front edgeto the center of the shoe has the clearance necessary to extend into astandard wafer cassette. Such a shoe typically can extend only partiallyinto the cassette, with the tip of the shoe adjacent the bottom centerof a wafer. Accordingly a vacuum suction is applied to the wafer toinsure positive retrieval and retention of the wafer on the shoe as thewafer is retracted from the cassette.

In order to permit the shoe to be extended even partially into the tightspacings of a standard wafer cassette, the shoe must be machined to bequite thin. But even a thin shoe inevitably is an undesireably close fitfor the tight clearances within a standard wafer cassette, even ifdimensioned to extend fully within a wafer cassette to engage a wafer.And simply relying on the thin retaining projection of such a thin shoealong with gravity to engage the wafer during transit may not providesufficient assurance that wafers will be consistently retained.

Neither may such a thin shoe provide sufficient assurance that the waferwill assume a consistent position during transit. For example, when theinner edge of the shoe comes into contact with a wafer, the edge of thewafer at one position sometimes becomes caught in the interface betweenthe inner edge of the shoe and the shoe bottom, so that the oppositeposition on the edge of the wafer is lifted upward. The wafer thus restsin a canted position in the shoe rather than centered and flat. When thewafer placement on the shoe is canted, wafer positioning on theprocessing support surface in the processing chamber is not consistentfrom wafer to wafer. This in turn can lead to a multitude of processingproblems, such as non-uniformity, plasma arcing, and damage to the waferand support surface.

Although the association of a vacuum pickup feature with such a shoe mayhelp with such problems, it is at best only a partial solution. It doesnothing to enhance the function of the shoe within evacuatedenvironments, nor the inevitable clearance problem presented inaccessing a standard cassette even with a thin shoe.

There is a need therefore for a wafer transfer apparatus and methodwhich would be equally efficient and effective both in a vacuum andambient environment, and in transferring wafers between bothenvironments. Also very desirable would be a wafer transfer apparatusand method that consistently assures proper holding and centering of thewafer under every condition and environment encountered during wafertransferral. Likewise very desirable would be a wafer processing systemwhich performs in an improved manner the transfer of wafers between astandard cassette and a transfer chamber at ambient pressure, as well asthe transfer of wafers between a transfer chamber when evacuated to asubatmospheric pressure and one or more evacuated wafer processingchambers. It would also be very advantageous to provide a wafer transferdevice that can withstand corrosive substances and elevatedtemperatures. The foregoing capabilities would be still more desirableif provided with a thin profile wafer transfer device with thecapability of smoothly accessing between the tight spaces of standardwafer storage cassettes, and which incorporated both a vacuum conduitand capacitive sensors.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a method of transferringsubstrates under conditions of varying pressure is provided. The methodincludes moving a substrate when the ambient pressure is relatively highby means of a substrate support provided with vacuum means for retainingthe substrate during such movement. The method also includes the step ofmoving the substrate when the ambient pressure is relatively low byremovably mounting to the support a substrate retainer; engaging andretaining the substrate by means of said support and retainer; andmoving the retained substrate by moving the substrate support.

More particularly a method is provided of transferring substratesbetween a first environment at generally ambient atmospheric pressureand a second subatmospheric processing environment, via a controlledintermediate environment which may be varied between generallyatmospheric pressure and a preselected subatmospheric pressure. Themethod includes moving a substrate from the first environment at ambientinto the controlled intermediate environment by means of a substratesupport provided with vacuum means for retaining said substrate duringits movement into and from the intermediate environment. The substrateis positioned at an intermediate storage position within saidintermediate environment. In order to move the substrate between thecontrolled intermediate environment and the subatmospheric processingenvironment, a substrate retainer is removably engaged to the substratesupport. The combined substrate support/retainer assembly engages andretains the substrate at its intermediate storage position and moves theretained substrate, with the intermediate environment now at apreselected subatmospheric pressure, between the intermediate positionand the subatmospheric processing environment.

In accordance with a further aspect of the invention, an apparatus isprovided for moving a substrate between a plurality of stations. Theapparatus includes a mechanical arm moveable between the variousstations; and first and second substrate supporting devices. The firstdevice is supported at the leading edge of said mechanical arm, and iscapable of supporting a substrate alone or in combination with thesecond substrate supporting device. The first device includes means forsecuring a substrate being transported by the arm and the first deviceagainst movement relative thereto while the mechanical arm is beingmoved between stations at generally ambient pressure. The secondsubstrate device is removably mountable to said first substrateretaining device, and secures a substrate while the arm is moved betweenstations at subatmospheric pressures.

More particularly, an apparatus is provided for accessing and processingsubstrates initially positioned within a primary substrate storage, suchas a standard wafer cassette, located outside of and adjacent theapparatus. The apparatus includes a housing defining one or moreprocessing chambers within its boundaries; a second substrate storagelocated within the housing, for example located within an intermediatetransfer chamber within the housing; and means for transferring asubstrate between the primary storage and the secondary storage, a wellas between the secondary storage and a processing chamber. This meansfor transferring includes a mechanical arm and generally flat blade atthe leading end of the arm. The blade includes suction means to hold asubstrate during transfer between primary storage and secondary storage.The means for transferring also includes a wafer tray stored within thehousing. The wafer tray is designed to be removably engageable with theblade and to hold a substrate during transfer between secondary storageand a processing chamber.

Accordingly a substrate may be transferred equally effectivelyregardless of whether it is to be moved within an environment atatmospheric pressure; or an environment at an evacuated subatmosphericpressure; or between both kinds of environments. When moving a substratethrough the ambient atmosphere, the vacuum feature of the substratesupport is activated to retain the substrate; while under subatmosphericconditions, the substrate support removably mounts the substrateretainer for positive retention of the wafer during this transferralunder these conditions for which the vacuum feature would not befunctional.

In the important application of semiconductor wafer processing, themethod and apparatus of the present invention is equally effective intransferring wafers between a standard cassette outside the processingsystem; a transfer chamber, a processing chamber, despite the fact thatthe wafers begin their processing in a standard wafer cassette atambient, and must be transferred into the very different subatmosphericprocessing environment of a processing chamber and back. The sametransfer apparatus functions equally effectively in both environments,as well as that of the variable environment of the intermediate wafertransfer chamber, which must vary from atmospheric to a subatmosphericpressure compatible with that of the processing chamber, in order topermit the transferral of wafers without compromising the environment ofa processing chamber.

The transfer apparatus including the blade with its suction means isdesigned to be very thin and optimized for extending between the narrowspacings of a wafer cassette in order to positively engage and support awafer, with the added security of the application of suction to thewafer while transferring at ambient pressure. The wafers transferredinto the transfer chamber are stored in an elevator with shelves morewidely spaced than a standard cassette.

The wafer supporting tray is retrieved by the blade from a rest positioninside the transfer chamber when transferring wafer under evacuated orsubatmospheric conditions. The tray and blade are configured withrespect to each other to form an interlocking assembly when the bladeengages and lifts the tray. The tray/blade assembly is configured tomatch a wafer and positively retain same when moved under a wafer withinthe storage elevator and transferring that wafer between the transferchamber and a processing chamber.

The tray can then be much thicker than the blade itself, and optimizedto have a size, contours, and significant retaining structure to insurethat wafers transferred under evacuated conditions are retained in avery positive and consistent manner, and as effectively as when theblade itself is working alone to transfer wafers at ambient pressure.Likewise, the construction and design of the blade is not compromised bythe need to consider the transfer of wafers under subatmosphericconditions.

In fact, the blade can be optimized as a thin flat ceramic for optionalmaneuvering between tight wafer shelf clearances of a cassette and forresistance to high temperatures, corrosive environments, and forstrength. The blade can also include capacitor sensors to aid in thecontrol of wafer transfer motions.

BRIEF DESCRIPTION OF THE DRAWING

In the accompanying drawings:

FIG. 1 is a plan view of a wafer processing apparatus including fourprocessing chambers, and a central wafer transfer chamber with its topplate partially cut away to show a wafer transfer arm assembly withinthe transfer chamber;

FIG. 2 is a side sectional view taken along line 2 of FIG. 1 of thecentral wafer transfer chamber, showing an intermediate wafer storageelevator inside the loadlock area; a slit valve defining a controlledaperture between the internal chamber environment and a wafer storagecassette external to the transfer chamber; and also showing the transferof wafers by the wafer transfer arm assembly between the intermediatewafer storage elevator and the external wafer cassette along theindicated direction;

FIG. 3 is a view similar to FIG. 2, showing the transfer of wafers bythe arm assembly between the wafer storage elevator and a wafer processchamber;

FIG. 4 is a perspective view of the intermediate wafer storage elevatorand an exploded view of the leading end of the wafer transfer armassembly, including a wafer transfer blade and removably engageablewafer support tray in accordance with the invention, as well as thecooperation between these elements;

FIG. 5 is a view similar to FIG. 4 showing the wafer support tray and awafer stored upon the intermediate storage elevator;

FIG. 6 is a side view through the intermediate wafer storage elevatorshowing the blade in position to lift the wafer tray from its restposition on the elevator, as well as showing wafers in position onshelves within the elevator;

FIG. 7 is a perspective view of the blade, and showing how a wafer ispositioned on the blade;

FIG. 8 is a perspective view of the blade with tray attached and placinga wafer above a wafer support pedestal inside a processing chamber withthe wafer in phantom, and with wafer support pins extending from thepedestal to lift the wafer from the blade tray;

FIG. 9 is a side sectional view taken across line 9--9 of FIG. 8 of thewafer transfer arm assembly, in position centrally over a wafer supportpedestal within a processing chamber including a blade and removablewafer tray, with the tray supporting a wafer shown in phantom, and withwafer support pins extended from the pedestal to engage the wafer;

FIG. 10 is a plan view of the upper half of the blade of the armassembly;

FIG. 11 is a plan view of the lower half of the blade of the armassembly; and

FIG. 12 is a plan view of a supporting T-bar of the arm assembly, bymeans of which the blade is attached to the robot arm of the armassembly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 generally shows a wafer processing system 10 including four waferprocessing chambers 12 and a central wafer transfer chamber 14 intowhich wafers 22 are transferred from a standard cassette 24. Thecassette is positioned outside and adjacent transfer chamber 14 within aprotective laminar flow hood 27 at generally ambient atmosphericpressure.

To accommodate the transfer of wafers into and from transfer chamber 14,a slit opening is provided in a wall of chamber 14 spaced fromprocessing chambers 12 and adjacent hood 27. A slit valve 25 ofconventional design controls the slit opening in the wall of thetransfer chamber. Slit valve 25, when closed, seals the chamber 14 fromthe ambient environment and from the laminar flow hood 27. Eachprocessing chamber 12/transfer chamber 14 interface likewise involves aslit opening similarly controlled by respective slit valves 28A, 28B,28C, and 28D.

As FIGS. 2 and 3 together with FIG. 1 generally show, transfer chamber14 is provided with a wafer transfer arm assembly 11 to effect thistransferral of wafers, whose novel features will be discussed below, butwhich generally includes a robot arm 16, a flat wafer carrying blade 18provided at the leading end of the robot arm 16, and a removable wafersupporting tray 40. Robot arm 16 moves only within a horizontal plane,but can extend forward from an elbow and is further capable of swingingaround in a 360 degree arc, while also maintaining any wafer beingtransported in a horizontal plane.

FIG. 2 shows this transferral of wafers between cassette 24 and transferchamber 24, while FIG. 3 shows the transferral of wafers betweentransfer chamber 14 and one of the processing chambers 12. Theprocessing chambers are continually kept at typically near-vacuumsubatmospheric pressure in accordance with typical wafer processingenvironments such as for etching, chemical vapor deposition or physicalvapor deposition. However, transfer chamber 14 may be either atatmospheric or at a low pressure comparable to that of a processingchamber 12. When wafers are being transferred between cassettes 24 andchamber 14 as in FIG. 2, transfer chamber 14 is at generally ambientpressure (chamber 14 preferably is also vented just prior to suchtransferrals to help in equalizing chamber pressure with ambient). Slitvalve 25 is open, and meanwhile valves 28A-28D remain closed, to protectthe subatmospheric processing environment of the processing chambers.

Once the transfer of wafers from cassette 24 into transfer chamber 14has been completed, slit valve 25 is closed, chamber 14 is pumped via apumping port 100 and pump 110 down to a subatmospheric pressure similarto that of the processing chambers while slit valves 28A-28D remainclosed. Once the pumping of transfer chamber 14 is completed to theproper subatmospheric pressure, wafers can then be transferred betweenthe processing chamber 12 and transfer chamber 14. Slit valves 28 arethen opened to permit the transfer of wafers between transfer chamber 14and the processing chambers, as illustrated in FIG. 3.

FIGS. 2 and 3 in conjunction with FIG. 1 also generally show the actionof robot arm 16, in particular blade 18 and tray 40 in moving wafersabout system 10. Flat blade 18 moves wafers, by means of an integralvacuum pick (which will be described in detail below), through slitvalve 25 between wafer cassette 24 and an intermediate wafer storageelevator 20 (also illustrated in partial perspective in FIGS. 4 and 5)inside chamber 14. The position of elevator 20 adjacent valve 25 andspaced from chambers 12 is best appreciated from FIGS. 1 and 2. Uponclosure of slit valve 25 to isolate transfer chamber 14 from theambient, and pumpdown of the transfer chamber to an evacuatedsubatmospheric pressure, blade 18 is moved into removable engagementwith tray 40, and both are utilized as a unit to securely engage andsupport wafers within the now-evacuated environment. In particular, asshown in FIG. 3 the blade/tray combination moves wafers between storageelevator 20 within the now-evacuated transfer chamber and the evacuatedsubatmospheric processing chambers.

Control, coordination and drive means for the slit valves, chambersevacuation, robot arm, and wafer storage elevator are well known in theart and need not be described in detail. The blade is provided withcapacitative sensors as will be described below, which are utilized in aknown manner to sense the proximity of a wafer or of the elevator, andto provide corresponding signals to guide such control means.

Turning now to the specifics of wafer transferral in system 10 incomplete detail, individual ones of wafers 22 are stored temporarily atrespective vertical shelf positions defined in elevator 20 and as shownin FIGS. 2, 5 and 6 while in transit between the outside environment, inthis case cassette 24, and one of the processing chambers. The storageelevator 20 is moved vertically up and down to align its various shelfpositions with the blade. When first loading system 10, wafer cassette24, which has been preloaded with a number of the semiconductor wafers22, is moved into position just outside the loading area 26 withinlaminar flow hood 27 by a conventional cassette transport means (notshown) which is capable of moving wafer cassette 24 vertically up anddown, as well as from side to side. At this time the robot arm is in aresting position whereby the leading end of blade 18 is retracted fromstorage elevator 20 and is aligned with slit valve 25.

In the illustrated preferred embodiment and as best shown in FIGS. 1 and2, elevator 20 is located in the path of movement of blade 18, so thatblade 18 passes through a shelf position of elevator 20 in its path toand from cassette 24. This presents no difficulty and is indeedadvantageous. As illustrated in the detail FIGS. 4 and 5, the spacingbetween respective shelf positions of elevator 20 is appreciably greaterthan the thickness of the thin blade, and also greater than the spacingbetween shelf positions of cassette 24, so that the blade can movehorizontally without risk of interference with elements of the elevatoror of the cassette. At the same time, the time and blade travelnecessary to move a set of wafers between the elevator and the cassetteis minimized.

Once the blade is adjacent a wafer within cassette 24, a vacuum pickcomprising one or more vacuum ports 29 adjacent to the distal end of theblade as illustrated in FIG. 7 applies suction to the blade. Thecassette 24 is then lowered so that the wafer is lifted by the blade 18clear of its shelf supports in the wafer cassette 24. With the waferthen securely held to the blade by vacuum suction, the robot arm thenretracts as best shown in FIG. 2 to remove the wafer from the cassette24 through open slit valve 25 to a position inside the transfer chamber14 in registration with a shelf position of elevator 20. The waferelevator then moves up to lift the wafer from the blade 18, whereuponthe vacuum holding the wafer to the blade is also released, and theblade is free to again move horizontally between shelf position ofelevator 20. The robot arm 16 moves the blade back towards the cassette24 in order to transfer another of wafers 22 in the same manner from thecassette 24 to the elevator. This transfer process is repeated until allthe wafers are transferred from the preloaded wafer cassette 24 to thewafer elevator 20. Of course this process is reversible, and blade 18can with the aid of its vacuum pick engage wafers at each shelf positionof elevator 20 and return same to shelf positions of cassette 24.

Because the transfer of wafers between the laminar flow hood 27 and thetransfer chamber 14 through the slit valve 25 is performed atsubstantially atmospheric pressure, the vacuum suction feature of theblade renders unnecessary any other wafer securing expedient, includingthe tray. An advantage of using only the blade to support wafers to andfrom the wafer cassette 24 is that the blade, being thin, can moreeasily maneuver within the tight spacings between wafers in thecassette.

As can be seen from FIG. 7, the wafer is supported with its center pointon the tip (distal end) of the blade. All that is required to keep thewafer on the blade is a suction applied at the interface between theblade and the wafer. The constructional details of the vacuum pick ofthe blade which provides the suction is described below with referenceto FIGS. 10 to 12. Once elevator 20 has been loaded with wafers fromcassette 24, the blade 18 is retracted into transfer chamber 14, slitvalve 25 is closed to seal off transfer chamber 14 from the outsideenvironment, and transfer chamber 14 is pumped down to a predeterminedsubatmospheric pressure, with both this pressure, and preferably thecomposition of the environment within the transfer chamber 14 beingcontrolled to be compatible with that of one of the processing chambers12A-D to which a wafer is to be transferred.

It will be noted that the wafer must now be moved between the transferchamber 14 and a processing chamber 12, both of which are now evacuatedenvironments at typically near-vacuum subatmospheric pressures; again,this movement is generally shown in FIG. 3. Clearly, the vacuum pickwhich served so well to secure the wafer as it was being moved fromambient pressure into the transfer chamber when the transfer chamber wasalso at ambient will no longer work in such evacuated environments.Accordingly, vacuum suction is not applied to the blade or wafer duringwafer transfer with the evacuated environment; rather, the robot arm 16now engages blade 18 with a removable tray 40. When not in use, tray 40is stored in a bottom location on storage elevator 20. FIG. 4illustrates the complimentary interlocking portions of the blade andtray; the particulars of their interlocking relationship will bediscussed in more detail below. FIG. 5 shows the tray in its reststorage position on the lowermost shelf of elevator 20. FIG. 8 bestillustrates the engaged tray-blade combination 40'.

The robot arm 16 operates to position the blade/tray 40' under aselected wafer 22 in the elevator as best shown in FIG. 6. As we havenoted, elevator 20 holds the individual wafers stored therein uponrespective shelves, each with greater separation than exhibited bycorresponding shelves of the standard cassette 24, and with enoughseparation to accommodate passage of tray/blade 40' with wafer. Eachwafer on the elevator is therefore easily accessed and removed by theblade/tray combination. The elevator 20 is moved downwards until thewafer 22 comes to rest on the blade/tray 40'. The wafer is then liftedoff the elevator shelf by blade/tray 40. As best seen in FIG. 6, thetray edges are shaped to match the curvature of the wafer, and aresloped inward, to enable a slightly offset wafer to slide into acentered position within the tray, and further to secure the waferagainst slippage during movement.

As illustrated by FIG. 3, the robot arm 16 then moves the blade/tray 40'and its accompanying securely-held wafer 22 into position toward apreselected one of processing chambers 12A-D. The appropriate one ofslit valves 28A-D (28B in FIGS. 2 and 3) is opened, and the robot armthen moves the blade/tray 40' with wafer 22 into the chamber 12 directlyabove and centered over wafer support plate 96. As illustrated in FIGS.8 and 9, two pairs of pins 95, which are spaced a distance wider thanblade/tray 40', move upward to lift wafer 22 off blade/tray 40'. Robotarm 16 retracts, and pins 95 are lowered into plate 96, so that wafer 22rests in a centered position atop plate 96. Because each wafer isproperly centered in blade/tray 40', each wafer is accurately centeredwith assurance on the wafer support plate for repeatable processing. Theblade/tray 40' then is withdrawn from the chamber 12, and the chamberslit valve 28 is closed. Thereupon wafer processing, such as chemicalvapor deposition or etch, commences within the processing chamber.

When wafer processing has been completed, the slit valve 28B is opened,wafer 22 is lifted upward by pins 95, the robot arm 16 moves theblade/tray 40' into the chamber 12 to a position underneath thenow-processed wafer 22, and removes the wafer from the processingchamber through the slit valve 28. The robot arm 16 may then move thewafer 22 into another processing chamber 12 or it may return the waferto the elevator 20. Eventually, however, the wafer completes itsassigned processing steps and the wafer must be returned to elevator 20,which rises upwards to lift the wafer 22 off the blade/tray 40'.

In moving a subsequent wafer to the processing chambers, the blade/tray40' is then withdrawn into transfer chamber 14, and elevator 20 ispositioned so that the blade/tray 40' can be moved into position underthe subsequent wafer in the elevator 20. Once again the elevator 20moves slightly downwards to bring the wafer in contact with theblade/tray 40', (i.e., until the wafer is lifted off its supports by itslower face catching on the blade/tray while the elevator moves downward)so that the robot arm 16 can remove the wafer from the elevator 20 andinsert it into a second processing chamber 12. Note that at all timesduring this transfer process the slit valve 25 is kept shut and anevacuated environment is continuously maintained inside the transferchamber 14.

The above process of transferring wafers using the blade/traycombination between the transfer chamber 14 and processing chamber 12through one of slit valves 28 is continued until all the wafers havebeen processed and returned to the elevator 20. At this time the wafersare ready for transfer back to the cassette 24. The blade/tray 40' movestoward the tray storage position situated on the elevator 20 andreverses the procedure of depositing wafers on the elevator. Theelevator moves upward to lift the tray 40 off the blade and into itsstorage position. Keeping the slit valves 28 and 25 closed, the transferchamber 14 is vented to approximately match the pressure of the laminarflow hood 27. When the pressures of the transfer chamber 14 and thelaminar flow hood 27 have equalized, slit valve 25 opens, and the robotarm 16 with blade 18 only transfers the wafers from the elevator 20 tothe cassette 24 using the vacuum suction feature of the blade to holdthe wafer in place while the robot arm 16 is in motion. This process isessentially similar to, but the reverse of, the originalcassette-to-elevator transfer process described above.

The details of the removable engagement of wafer tray with blade 18 arebest appreciated with the aid of FIGS. 4-6. When the tray 40 is notpositioned on the blade as a blade/tray combination 40', the tray ispositioned at the base of the elevator 20 on two pairs of positioningpins 44a and 44b as in FIG. 5. The leading or distal positioning pins44a each have a flat topped surface on which the tray 40 rests. Theproximal pins 44b, however, act as centering pins and each have aprotrusion 46 formed on its top. These protrusions 46 fit intocomplementally sized sockets formed into the bottom of the tray. As aresult of this configuration the tray always is caused to rest in thesame lowest shelf position when supported on the base of the elevator20.

As can be seen from FIG. 4, the tray 40 includes distal and proximalends, 54 and 56 respectively, both of which have semicircular cutoutswith sloping faces 54', 56' formed therein. These semicircular cutoutsare sized to be approximately equivalent to the wafer size and to followthe curvature of a wafer. As shown in FIG. 6, the physical constructionof the tray 40 allows the tray to receive the semiconductor wafer 22 asthe elevator moves downward, and automatically center the wafer in thetray by enabling a slightly offset wafer to slide down the sloping facesinto place. Since the wafer is positioned inside the raised distal andproximal ends, the wafer is held in place when the robot arm 16 moves.

Before the transfer of wafers from the elevator to the processingchambers 12 commences, the blade 18 is inserted underneath the tray 40as shown in FIG. 4. The blade 18 has a pair of centering pins 48 nearwhere the blade 18 is connected to the wafer robot arm 16. These pins 48are sized to fit into sockets 50 in the tray 40 and, in so doing, ensurethat the tray 40 is correctly positioned on the blade 18. As illustratedin FIGS. 4 to 6, to further ensure this positioning, the tray 40 has itslong sides 52 designed to receive the blade 18 between them so that theyoverhang the blade 18 and keep the tray 40 in position. Once the blade18 has been positioned under the tray 40 so that the centering pins 48are directly underneath the sockets 50 of the tray, the elevator 20 ismoved downwards until the tray 40 is engaged by the blade 18 and israised clear of its supporting pins 44a and 44b. The happening of thisevent is sensed by means of capacitive sensors within blade 18, theconfiguration of which will be described further below with reference toFIGS. 10 and 11.

When processing has been completed and the wafers have been moved backto storage elevator 20, robot arm 16 moves blade/tray 40' to its storagelocation at the lowest shelf of storage elevator 20 and deposits tray 40thereon.

The constructional details of the two part ceramic blade 18 are morefully given in FIG. 10 which shows the upper half of the blade, FIG. 11,which shows the lower half of the blade, and FIG. 12 which is a planview of a supporting T-bar which is located underneath the blade 18 andwhich connects it to the robot arm 16.

The upper half 18' of the blade 18 as shown in FIG. 10 is constitutedpreferably by an alumina ceramic plate. The central portion of the bladehas a rectangular hole 60 formed therethrough and at the blade's leadingend two oblate apertures 29 are formed through it. The purpose of theseapertures will be described later with reference to FIG. 11. Also shownare four countersink holes 64 formed through the upper half of the blade18'. In addition, this figure shows two generally rectangular shapedelectrodes 66, 68 formed thereon by a gluing and firing process, whichterminate in electrical connectors 66' and 68' respectively. Each ofthese two rectangles 66, 68 defines the upper half of a capacitivesensor, one of which is used to determine whether or not the tray hasbeen engaged by the blade, and the other of which is used to determinewhether or not the wafer has been engaged by the blade.

In FIG. 11 the lower half 18" also made of alumina ceramic, is shown toinclude an aperture 60' corresponding to the aperture 60 shown in FIG.10. In addition, holes 64', corresponding to holes 64 in FIG. 10, passthrough the blade lower half 18". This figure also shows a vacuumchannel 70 which has been formed in the surface of the blade lower half18". At the one end of the generally rectangular shaped vacuum channel,two extensions 72 are formed.

When the upper half 18' of the blade 18 is placed onto the lower half18" of the blade, the rectangular apertures 60 and 60' are inregistration with one another to define an aperture which passes throughthe blade 18. Similarly, the holes 64 and 64' are in registration withone another to permit fastening screws (not shown) to pass through themso that the blade can be secured onto the mounting T-bar illustrated inFIG. 12. When the two halves of the blade are placed together, theextensions 72 lie directly below the oblate apertures 29. As a resultthe vacuum channel 70 is able to communicate directly with the uppersurface of the blade 18 so that, when a wafer is received thereon and avacuum is applied in the channel 70, this vacuum can be transferred tothe interface between the wafer and the blade 18.

The upper and lower halves of the blade are sintered together to form athin blade of approximately 0.050" of high strength and ability towithstand potentially high wafer temperatures. A thin blade is desiredbecause it can more easily move within the clearance between wafers on astandard cassette, which can be as little as approximately 0.20" (5 mm)wide.

FIG. 11 also shows two generally rectangular shaped electrodes 76, 78which correspond in size and position to the similarly-shaped electrodes66 and 68 shown in FIG. 10. These pads 76, 78 are the lower halves ofthe two capacitive sensors which are respectively used to determinewhether or not the wafer tray 40 or the wafer 22 itself has been loadedonto the blade.

The upper pad 66 has the same area and shape as its corresponding pad76. The same applies to the upper pad 68 and its corresponding pad 78.When the upper and lower halves of the blade 18 are sintered together,their respective electrodes are separated by about 0.001" (25μ). Thismeans that two capacitive sensors, one defined by pad 66 and pad 76 andthe other by pad 68 and pad 78, are formed in the assembled blade. Itshould be noted that these capacitive sensors are not contact sensors,but rather operate to determine a change in capacitance as the blade andthe wafer tray (or blade and wafer) move closer together. The sensorsare calibrated so that they signal the presence of a wafer or the trayonly when the wafer or tray are properly centered on the blade.

Finally, FIG. 11 shows a circular hole 80 formed through the lower half18" of the blade 18. This hole 80 provides a direct communication portbetween the vacuum channel 70 and a similarly sized hole 82 formedpartway into the T-bar 84 illustrated in FIG. 12. The hole 82 in theT-bar 84 is connected to a vacuum source (not shown) by means of aconduit 86 and a vacuum port 88. This vacuum port communicates with thevacuum source through a channel formed in the robot arm 16, which isshown in broken lines in this figure. The T-bar is also shown to includefour threaded holes 64". These threaded holes correspond to the holes 64and 64' shown in FIGS. 10 and 11 and receive fasteners which passthrough the upper and lower blades 18' and 18" to secure the blade 18onto the T-bar 84. As can be seen from this figure the T-bar has thecentering pins 48 mounted on it. These pins pass directly throughapertures 48" formed in both the upper and lower halves of the blade 18to protrude beyond it. In addition, FIG. 12 shows two studs 90protruding vertically upward out of the T-bar 84. These studs fit intoholes 90' formed in the upper and lower halves of the blade 18 and actto center the blade onto the T-bar when the T-bar and the blade 18 arebeing secured together by means of the screws passing through apertures64.

Conventional wafer carrying blades, which are made of plastic, metaland/or silicon, cannot achieve the great thinness of the present blade,as these previously used materials lose their structural strength underthe high-heat operating conditions of the wafer processing reactors. Itis only with the use of materials such as alumina ceramic, which havehigh structural rigidity even under high temperatures, that a blade canbe made as thin as the blade of the invention (typically approximately0.05").

The carrying blade and tray combination of this invention has a numberof further important advantages over the prior art devices. Since thetray is removable, a trayless thin blade with vacuum suction can be usedfor transferring wafers between the cassette and elevator. As a result,it enables reliable wafer transfer despite the small gaps between thewafers in the cassette. Then, when wafer transfer within an evacuatedenvironment is required, thereby precluding the use of vacuum suction onthe blade, the tray can be easily interlocked with the blade forreliable, slip-free wafer transfer within a vacuum environment. Althoughnot necessary for the practice of the other aspects of the invention,the desirable two part ceramic construction of the blade is much easierand cheaper to fabricate than earlier blades and, once in use, the bladeretains its structural strength and its different halves do not separatefrom one another.

While the invention has been particularly shown and described withreference to certain preferred embodiments, it will be understood bythose skilled in the art that various alterations and modifications inform and in detail may be made therein. Accordingly, it is intended thatthe following claims cover all such alterations and modifications as mayfall within the true spirit and scope of the invention. It should alsobe noted first that although the invention is described in the contextof semiconductor wafers, this invention is not limited to use withsemiconductor wafers; rather, it envisions use with any and allsubstrates which are relatively flat and are to be transferred and heldin both ambient and evacuated environments.

We claim:
 1. A method of transferring substrates under conditions ofvarying pressure, comprising the steps of:(a) moving a substrate whenthe ambient pressure is relatively high using a substrate supportprovided with at least one vacuum port capable of engaging anundersurface of said substrate during such movement; and (b) moving asubstrate when the ambient pressure is relatively low by:(i) removablymounting to said substrate support a substrate support tray capable ofengaging and supporting a bottom surface of said substrate; (ii) placingsaid substrate on said substrate support tray by engaging and supportingsaid bottom surface of said substrate with said substrate support trayand (iii) moving said retained substrate by moving said substratesupport and said substrate support tray mounted on said substratesupport.
 2. A method as in claim 1 in which the substrate is moved whilebeing maintained in a horizontal position, and in which one of saidsubstrate support tray and said at least one vacuum port secure saidsubstrate against lateral movement with respect to said substratesupport when said substrate is being moved.
 3. A method as in claim 1 inwhich said substrate support comprises a generally flat blade.
 4. Amethod of transferring substrates to and from a first station wheresubstrates are supported in a relatively narrowly spaced array, and toand from a second station where substrates are supported in a widelyspaced array, comprising the steps of:(a) moving a substrate held insaid first station by engaging an undersurface of said substrate with agenerally flat blade provided with at least one vacuum port forretaining said under surface of said substrate to said blade during suchmovement; and (b) moving a substrate held in said second station by:(i)removably mounting a substrate support tray onto said blade, (ii)lifting said substrate with said substrate support tray by bringing saidsubstrate support tray into contact with said under surface of saidsubstrate, and (iii) moving said substrate by moving said blade havingsaid substrate support tray mounted thereon.
 5. A method as in claim 4which further includes the step of moving a substrate held in saidsecond station to a wafer processing station and back upon completion ofprocessing at such wafer processing station.
 6. A method as in claim 5in which a substrate is moved between said first station and said secondstation under generally atmospheric pressure conditions; and in which asubstrate is moved between said second station and said wafer processingstation under subatmospheric conditions.
 7. A method as in claim 6 inwhich said second station is provided within a pressure controllablechamber capable of being evacuated to attain said subatmosphericconditions, and also capable of being vented to be under generallyatmospheric pressure conditions.
 8. A method as in claim 4 in which saidnarrowly-spaced array in said first station is provided by a wafercassette.
 9. A method of transferring substrates between a firstenvironment at generally ambient atmospheric pressure and a secondsubatmospheric processing environment, via a controlled intermediateenvironment which may be varied between generally atmospheric pressureand a preselected subatmospheric pressure, comprising the steps of:(a)moving a substrate from said first environment into said controlledintermediate environment, with said intermediate environment being atgenerally ambient atmospheric pressure, using a substrate supportprovided with at least one vacuum port which engages an under surface ofsaid substrate to secure said substrate to said substrate support whilesaid substrate support moves said substrate into said intermediateenvironment, said substrate being positioned at an intermediate storageposition within said intermediate environment; (b) removably engaging tosaid substrate support a substrate retainer tray having tray edgesshaped to match the edges of said substrate to laterally retain saidsubstrate thereon; and (c) lifting said substrate by engaging said undersurface of said substrate with said substrate retainer tray while saidtray edges contact said edges of said substrate to laterally retain saidsubstrate on said substrate retainer tray at said intermediate storageposition; and (d) moving said retained substrate, with said intermediateenvironment at a preselected subatmospheric pressure, from saidintermediate position into said second subatmospheric processingenvironment, using said substrate support and said substrate retainertray.
 10. A method of transferring one or more substrates between aprocessing chamber maintained at a subatmospheric processingenvironment, a primary substrate storage region generally at ambientpressure, and a secondary substrate storage region provided with acontrollable environment variable between ambient and subatmosphericpressures, the method comprising the steps of:(a) providing within saidsecondary storage region a mechanical arm, a generally flat blade at aleading end of said mechanical arm, at least one vacuum port beingdefined by said flat blade, and a substrate supporting tray; (b)controlling said secondary substrate storage to generally ambientatmospheric pressure; (c) engaging an under surface of a substrate insaid primary substrate storage with said blade, and retaining said undersurface of said substrate to said blade by activating said at least onevacuum port; (d) transferring a number of retained substrates betweensaid primary substrate storage and said secondary substrate storage atgenerally ambient atmospheric pressure, using said flat blade; (e)mounting said substrate supporting tray onto said flat blade; (f)adjusting the environment of said secondary substrate storage to asubatmospheric pressure; (g) engaging said under surface of saidsubstrate located in said secondary substrate storage with saidsubstrate supporting tray, and laterally retaining said substrate onsaid substrate supporting tray using tray edges on said substratesupporting tray shaped to match the edges of said substrate to laterallyretain said substrate thereon; and (h) transferring said retainedsubstrate between said secondary storage at said subatmosphericpressure, and said processing chamber using said substrate supportingtray, said flat blade, and said mechanical arm.